UDP packets are getting dropped because of invalied checksum though checksum offload is enabled in kernel-2.6.32

[Murali Annamneni]

Hi Kristof,

Thanks for your quick reply.

I have made changes to my code suchthat before leaving from post hook(i.e. before entering into the ethernet device),  I set udp & ip checksum's as "0". 
Still packets are getting dropped. I found that oflload engine is not computing the checksum.
Do you know anything about  checksum offload feature is corrected in kernel-2.6.32 ?


Thanks & Regards
Murali Annamneni


-----Original Message-----
From: Kristof Provost [mailto:kristof at sigsegv.be] 
Sent: Thursday, July 19, 2012 6:53 PM
To: Murali Annamneni
Cc: kernelnewbies at kernelnewbies.org
Subject: Re: UDP packets are getting dropped because of invalied checksum though checksum offload is enabled in kernel-2.6.32

On 2012-07-19 18:28:57 (+0530), Murali Annamneni <a.murali at hcl.com> wrote:
> Hi All,
> 
> I have written a kernel module using netfilter framework. In my code I'll do some modifications to the header & payload parts of UDP packet in pre & post hooks.
> After modifying the udp packet, I'm computing ip & udp checksum. This computation is consuming some cpu cycles.
> Now, I want to offload this computation to the Ethernet device. For this I have commented the checksum computation in my code and enabled the checksum offload parameters of my Ethernet device.
> While testing, I found that kernel is dropping the packets because of invalid checksums for ip & udp packets.
> I have few doubts regarding checksum offload
> 
Wild guess: try setting the checksum to zero before sending the packet.
It's possible that the offload engine won't recompute checksums unless they're zero (i.e. it might assume non-zero checksums are already correct).

Regards,
Kristof



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